Method and apparatus for efficient fast retraining of ethernet transceivers

ABSTRACT

A method of operation for an Ethernet transceiver is disclosed. The method includes entering a fast retrain sequence of steps. The fast retrain sequence of steps includes transferring two-level symbols to a link partner; and directly following transferring of the two-level symbols, transferring multi-level symbols to the link partner, the multi-level symbols having greater than two symbol levels.

TECHNICAL FIELD

The disclosure herein relates to communications systems, and morespecifically to high-speed Ethernet systems and methods.

BACKGROUND

Modern high-speed Ethernet protocols, such as 10GBASE-T and NBASE-T,rely on adaptive filters and digital signal processing circuitry toaddress noise that may affect the Ethernet link. Prior to operation ofthe link in an “online” mode, the link undergoes autonegotiation and afull training sequence to place the link in an initial state that'scapable of overcoming the noise. While operating in the “online” mode,the link may briefly go “offline” due to extraneous alien crosstalk orother noise. Fast retrain sequences may bring the link back online in avery short period of time without the need to carry out a full trainingsequence.

Conventional fast retraining sequences for high-speed Ethernet linksgenerally take approximately 30 milliseconds. For many applications,such time durations for retraining functions may be undesirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure are illustrated by way of example, and notby way of limitation, in the figures of the accompanying drawings and inwhich like reference numerals refer to similar elements and in which:

FIG. 1 illustrates a high-level transmitter/receiver (transceiver)channel architecture for an NBASE-T Ethernet transceiver.

FIG. 2 illustrates one embodiment of an FIR (finite impulse response)implementation of the correlated noise canceller for use with thereceiver circuits of FIG. 1.

FIG. 3 illustrates a flow diagram of one embodiment of a method ofcancelling correlated noise.

DETAILED DESCRIPTION

FIG. 1 is a block diagram illustrating one embodiment of a communicationsystem 100. The system includes a first transceiver integrated circuit(IC) or chip 102 and a second transceiver chip 104 that can communicatewith each other. The first transceiver 102 includes “transceivercomponents” including one or more transmitters TX_(A)-TX_(D) and one ormore receivers RX_(A)-RX_(D). Similarly, the second transceiver 104includes various transceiver components including one or moretransmitters TX_(E)-TX_(H) and one or more receivers RX_(E)-RX_(H). Thetransmitters TX_(A)-TX_(H) shown in FIG. 1 can be considered individual“transmitters,” as typically referenced herein, or can be consideredindividual transmitter channels which a transmitter block within thetransceiver can independently transmit signals on. Similarly, receiversRX_(A)-RX_(H) can be considered individual “receivers,” as typicallyreferenced herein, or can alternately be considered individual receiverchannels which a receiver block within the transceiver can independentlyreceive signals on. The transmitters and receivers are connected to oneor more components (not shown) of a computer system, device, processor,or other “controller” associated with each respective transceiver whichwants to communicate data over the communication network. For example,the transmitters receive data and control signals from the controllerconnected to the first transceiver 102 in order to send the data overthe network to other transceivers and controllers, while the receiversreceive data from other transceivers and controllers via the network inorder to provide the data to the controller connected to the firsttransceiver 102.

The first transceiver chip 102 can communicate with the secondtransceiver chip 104 over one or more communication channels of acommunication link 106. In one embodiment, such as one similar to the10GBASE-T Ethernet standard, four communication channels are provided onthe communication link 106, each channel including a twisted pair cable.Thus, in that standard, there are four transmitters TX and fourcorresponding receivers RX provided in each of the transceivers 102 and104, each transmitter associated with one of the local near-endreceivers in the same transceiver, and each such transmitter/receiverpair dedicated to one channel used for duplex communication. Atransmitter/receiver pair in the first transceiver 102 communicatesacross a channel of the link 106 to a far-end transmitter/receiver pairin the second transceiver 104. A transmitter TX and a receiver RX thatare connected to the same channel/link, or two transceivers connected bythe communication link 106, are considered “link partners.” Inaccordance with the 10GBASE-T standard, the transmitters cooperate withprecoders (not shown), and more specifically Tomlinson-Harashimaprecoders (THP) that apply THP coefficients to transmit signals topre-compensate for channel imperfections and expected noise/distortion.

An interface 108 can be provided in the first transceiver chip 102 andan interface 110 can be provided in the second transceiver chip 104 toallow data transmissions between the transceivers to be routed to theappropriate transceiver blocks. For example, the interfaces 108 and 110can include transformers, and circuitry used for directing signals ordata (alternatively, some or all circuitry can be included in othercomponents, such as transmitters TX and receivers RX).

For one embodiment, the interface 108 for each chip includes traininglogic 112 that manages and controls rapid retrain operations, as morefully explained below. The training logic associated with the interface108 may be shared amongst the channels, or provided as separateresources for each of the transceiver channels.

Further referring to FIG. 1, each of the receiver circuits includes acorrelated noise canceller, such as canceller CNC_(A) for receiverRX_(A). As more fully described below, the correlated noise cancellersenable faster retrain sequences by enabling a receive side of a link tocancel interference resulting from alien crosstalk or noise withoutresorting to a need for changing and exchanging precoder THPcoefficients during the fast retrain sequence.

Referring to FIG. 2, in one embodiment, each correlated noise canceller200 is realized as an adaptive filter that continually adapts, and moreparticularly a finite impulse response (FIR) filter. While the structureand operation of FIR filters are generally well-known to those skilledin the art, FIG. 2 illustrates a high-level implementation that includesa plurality of delay elements D0-Dn, for delaying respective datasamples. A number of taps T0-Tn corresponding to the delay elements arerespectively coupled to delay output nodes. The taps receive respectiveweights or coefficients h0-hn+1 that represent factors applied to asequence of data samples. A summing circuit 202 receives weightedsamples, averages them, and outputs a filtered value.

The length of the adaptive filter, or number of samples to evaluate andaverage to determine an output, is selectable for a user depending onthe application involved. Implications involving amplifier gain maywarrant the use of only one sample (weighted with a gain or tap valueof 1) or many samples (weighted with a corresponding number of low-gainvalues). Generally, however, the longer the filter, the less correlationin the noise, and correspondingly less predictability. As discussedbelow, high gain values may have a tendency to promote errorpropagation, and possibly cause filter non-convergence.

A variety of algorithms may be employed to derive values for theselected number of tap weights, or coefficients. Least mean squares(LMS) and minimum mean square error are 2 of well known algorithms. Onevariant to adapt the coefficients is to “regularize” the filter byutilizing a least squares algorithm, as is well-known in the art. As anexample, one might use the least squares algorithm to find a solutionthat minimizes gain for the coefficients. Another possible algorithm toemploy is the least mean squares method, which provides a leakage termthat allows each adaptation step to “leak” a little bit away from theresult so that the output doesn't grow too large too fast (thus,avoiding a high gain situation). In a further embodiment, the taps mayhave programmable saturation levels to maximize tap gain levels. Furtherdetails regarding the circuitry and function of one embodiment of acorrelated noise canceller may be found in U.S. Pat. No. 8,861,663,titled “Correlated Noise Canceller For High-Speed Ethernet Receivers”,filed Jan. 13, 2012, assigned to the Assignee of the present disclosure,and expressly incorporated by reference herein in its entirety.

In operation, the receiver circuits described above take advantage ofthe fact that correlated noise, such as that typically associated withRFI, may be predictable in many cases based on historical statistics.Thus, by sampling correlated noise over time, and assuming that thesampled noise is correlated between samples, evaluating the history ofthe noise may be a reliable predictor of future noise. Further, bypredicting the future noise, it can be filtered/canceled from a currentsample to enhance the signal fidelity of the system. Note that the useof the term “cancellation” is not meant to require a completecancellation of a target signal, but rather a significant mitigation orreduction of the target signal that provides the advantages notedherein.

Referring now to FIG. 3, one embodiment of a fast retrain method,generally designated 300, provides multiple fast retrain modes ofoperation in an Ethernet transceiver that can enable a fast linkrecovery for a link affected by a change in noise or some otherundesirable distortion characteristic. In response to detecting a dropin link quality, such as a minimum SNR or rate of LDPC error frames orother LDPC statistics, a fast retrain sequence may be initiated, at 302.As noted above, a fast retrain is a brief link training sequence thatresets a minimal number of link parameters to allow for changes in linkparameters without necessarily requiring the link to go offline for afull auto-negotiation and training cycle, which may take a few seconds.

Further referring to FIG. 3, in one embodiment, the fast retrainsequence may first provide a PAM2-THP transmission sequence, at 304.During the PAM2-THP sequence, a PAM-2 training signal is transmittedwith precoding, using appropriate coefficients and other parameters. Theprecoding applied to the PAM-2 signal allows the far-end receiver toretune its filters for the potential difference between optimaloperating points with and without the precoding, so that the sameadaptation can be used during normal transmission. For some embodiments,the PAM2-THP step may be omitted entirely, such as disclosed incopending U.S. patent application Ser. No. 16/362,479; titled “RAPIDTRAINING METHOD FOR HIGH-SPEED ETHERNET”, filed Mar. 22, 2019, assignedto the Assignee of the present disclosure, and expressly incorporatedherein by reference in its entirety. Following the PAM2-THP sequence, adetermination may be made as to whether the receiver can recover thelink by itself, without a THP coefficient exchange, at 306. Thedetermination may be based on an evaluation of how far off the newprecoder coefficients are from prior coefficients, and so forth.

With continued reference to FIG. 3, in the case where the receivercannot recover the link by itself, then newly updated THP coefficientsmay be exchanged between link partners and the receiver synchronizedwith precoded transmission data from the far-end link partnertransmitter, at 308. In other embodiments, other signal types,modulations, or formats can be used for training signals as appropriate.

Further referring to FIG. 3, following the THP coefficient exchange, atransition period occurs, at 312, to allow the link to transition fromtransmitting the precoded PAM-2 signals, to transmitting a multi-PAMtype of signal having characteristics more closely resembling thoseactually used during nominal operation, such as a PAM-16 or DSQ128signal. After the transition period, actual PAM-16 training signalsencoded with the THP coefficients, and modulated via a DSQ128 type ofmodulation may be transmitted, at 314. After sending the PAM-16 trainingsignal for an appropriate time to adapt the filters, equalizers, timing,and other components, the training sequence ends and normal operationbegins, in which actual data is transmitted and received over the linkas PAM-16 signals.

While the “legacy” fast retrain mode described above works well for itsintended applications, the time expended to carry out the coefficientexchange step at 308 may take up a significant portion of the timingbudget allotted to each fast retrain sequence.

With the above in mind, should the receiver be able to recover the linkby itself, at step 306, one embodiment provides for a reduced-processingfast retrain mode, beginning at 316, that omits the legacy THPcoefficient exchange step. This is possible due to the filteringactivities of the correlated noise cancellers, which are receive-sideadaptive filters that do not utilize transmit-side THP coefficients.This results in a significant training time reduction. Note that in someembodiments, a separate “legacy” mode of fast retraining may not beprovided as a separate mode from the reduced-processing training mode.

With further reference to FIG. 3, for some embodiments, and afterconfirming that the signal quality meets a certain threshold, acommunication between link partners in the reduced-processing trainingmode is carried out, at 316. This is carried out so that the partnersare synchronized in knowing that the reduced-processing fast retrainmode will be followed, instead of the legacy mode. This communicationmay be made by, for example, an InfoField protocol that includes one ormore bits flagging use of the reduced-processing training mode.

Following the communication step, at 316, and an associated transitiontime, at 320, the reduced-processing mode goes straight to a PAM-16THPstep, at 322, to run a training sequence of THP-encoded PAM-16 signals.

By eliminating the THP coefficient exchange step, a fast retrainsequence may be shortened significantly. Since fast retrains may occuroften during normal operation of the link, repetitive fast retrainsequences of shorter duration saves appreciable time.

Those skilled in the art will appreciate the benefits and advantagesprovided by the embodiments described herein. Reducing fast retrainingsteps for high speed Ethernet transceivers minimizes link downtime whileimproving processing efficiency.

When received within a computer system via one or more computer-readablemedia, such data and/or instruction-based expressions of the abovedescribed circuits may be processed by a processing entity (e.g., one ormore processors) within the computer system in conjunction withexecution of one or more other computer programs including, withoutlimitation, net-list generation programs, place and route programs andthe like, to generate a representation or image of a physicalmanifestation of such circuits. Such representation or image maythereafter be used in device fabrication, for example, by enablinggeneration of one or more masks that are used to form various componentsof the circuits in a device fabrication process.

In the foregoing description and in the accompanying drawings, specificterminology and drawing symbols have been set forth to provide athorough understanding of the present invention. In some instances, theterminology and symbols may imply specific details that are not requiredto practice the invention. For example, any of the specific numbers ofbits, signal path widths, signaling or operating frequencies, componentcircuits or devices and the like may be different from those describedabove in alternative embodiments. Also, the interconnection betweencircuit elements or circuit blocks shown or described as multi-conductorsignal links may alternatively be single-conductor signal links, andsingle conductor signal links may alternatively be multi-conductorsignal links. Signals and signaling paths shown or described as beingsingle-ended may also be differential, and vice-versa. Similarly,signals described or depicted as having active-high or active-low logiclevels may have opposite logic levels in alternative embodiments.Component circuitry within integrated circuit devices may be implementedusing metal oxide semiconductor (MOS) technology, bipolar technology orany other technology in which logical and analog circuits may beimplemented. With respect to terminology, a signal is said to be“asserted” when the signal is driven to a low or high logic state (orcharged to a high logic state or discharged to a low logic state) toindicate a particular condition. Conversely, a signal is said to be“deasserted” to indicate that the signal is driven (or charged ordischarged) to a state other than the asserted state (including a highor low logic state, or the floating state that may occur when the signaldriving circuit is transitioned to a high impedance condition, such asan open drain or open collector condition). A signal driving circuit issaid to “output” a signal to a signal receiving circuit when the signaldriving circuit asserts (or deasserts, if explicitly stated or indicatedby context) the signal on a signal line coupled between the signaldriving and signal receiving circuits. A signal line is said to be“activated” when a signal is asserted on the signal line, and“deactivated” when the signal is deasserted. Additionally, the prefixsymbol “/” attached to signal names indicates that the signal is anactive low signal (i.e., the asserted state is a logic low state). Aline over a signal name (e.g., ‘<signal name>’) is also used to indicatean active low signal. The term “coupled” is used herein to express adirect connection as well as a connection through one or moreintervening circuits or structures. Integrated circuit device“programming” may include, for example and without limitation, loading acontrol value into a register or other storage circuit within the devicein response to a host instruction and thus controlling an operationalaspect of the device, establishing a device configuration or controllingan operational aspect of the device through a one-time programmingoperation (e.g., blowing fuses within a configuration circuit duringdevice production), and/or connecting one or more selected pins or othercontact structures of the device to reference voltage lines (alsoreferred to as strapping) to establish a particular device configurationor operation aspect of the device. The term “exemplary” is used toexpress an example, not a preference or requirement.

While the invention has been described with reference to specificembodiments thereof, it will be evident that various modifications andchanges may be made thereto without departing from the broader spiritand scope of the invention. For example, features or aspects of any ofthe embodiments may be applied, at least where practicable, incombination with any other of the embodiments or in place of counterpartfeatures or aspects thereof. Accordingly, the specification and drawingsare to be regarded in an illustrative rather than a restrictive sense.

We claim:
 1. A method of operation for an Ethernet transceiver, themethod comprising: entering a fast retrain sequence of steps, the fastretrain sequence of steps including transferring two-level symbols to alink partner without transferring Tomlinson-Harashima Precoder (THP)coefficient information; and directly following transferring of thetwo-level symbols, transferring multi-level symbols to the link partner,the multi-level symbols having greater than two symbol levels.
 2. Themethod according to claim 1, wherein the transferring two-level symbolsto a link partner comprises: selectively transferring uncoded two-levelsymbols to a link partner; and selectively transferring coded two-levelsymbols to the link partner.
 3. The method according to claim 2, whereinthe coded two-level symbols are encoded in accordance withTomlinson-Harashima Precoder (THP) coefficients.
 4. The method accordingto claim 1, further comprising: informing the link partner to proceeddirectly to transferring the multi-level symbols following thetransferring of the two-level symbols.
 5. The method according to claim4, wherein the informing is carried out via an InfoField protocol. 6.The method according to claim 1, wherein: the two-level symbols comprisePAM-2 symbols; and the multi-level symbols comprise PAM-16 symbols. 7.The method according to claim 1, further comprising: during the fastretrain sequence of steps, cancelling correlated noise received by areceiver with a correlated noise canceller.
 8. An integrated circuit(IC) Ethernet transceiver chip comprising: receiver circuitry includingcorrelated noise canceller circuitry; and training logic to control afast retrain sequence, the fast retrain sequence including a transfer ofuncoded two-level symbols to a link partner; a transfer of codedtwo-level symbols to the link partner following the transfer of theuncoded two-level symbols without transferring Tomlinson-HarashimaPrecoder (THP) coefficient information; and directly following thetransfer of the coded two-level symbols, the training logic to control atransfer of multi-level symbols to the link partner, the multi-levelsymbols having greater than two symbol levels.
 9. The IC Ethernettransceiver chip of claim 8, wherein the transfer of two-level symbolscomprises: a transfer of uncoded two-level symbols to a link partner;and a transfer of coded two-level symbols to the link partner.
 10. TheIC Ethernet transceiver chip according to claim 9, wherein: the codedtwo-level and multi-level symbols are encoded in accordance with aTomlinson-Harashima Precoder (THP) encoding.
 11. The IC Ethernettransceiver chip according to claim 8, wherein: the two-level symbolscomprise PAM-2 symbols; and the multi-level symbols comprise PAM-16symbols.
 12. The IC Ethernet transceiver chip according to claim 8,realized as an NGBASE-T IC Ethernet transceiver chip.
 13. A method ofoperation for an Ethernet transceiver, the method comprising: for afirst mode of operation, performing a training sequence by transferringtwo-level symbols to a link partner; and directly following transferringof the two-level symbols, transferring multi-level symbols to the linkpartner, the multi-level symbols having greater than two symbol levels;and for a second mode of operation, performing the training sequence bytransferring uncoded two-level symbols to a link partner; exchangingupdated precoder coefficients with the link partner, transferring codedtwo-level symbols to the link partner, the coding consistent with theupdated precoder coefficients, and transferring multi-level symbols tothe link partner, the multi-level symbols encoded consistent with theexchanged updated precoder coefficients and having greater than twosymbol levels.
 14. The method according to claim 13, wherein for thefirst mode of operation, the transferring two-level symbols to a linkpartner comprises: selectively transferring uncoded two-level symbols toa link partner; and selectively transferring coded two-level symbols tothe link partner.
 15. The method according to claim 13, wherein aselection between using the first mode of operation or the second modeof operation is based on a signal quality parameter.
 16. The methodaccording to claim 13, wherein the precoder encoding comprises: aTomlinson-Harashima Precoder (THP) encoding.
 17. The method according toclaim 13, wherein the first mode of operation further comprises:informing the link partner to proceed directly to transferring themulti-level symbols following the transferring of the coded two-levelsymbols to the link partner.
 18. The method according to claim 17,wherein the informing is carried out via an InfoField protocol.